This invention relates to the preparation of 1,3-propanediol-based polyesters. In one aspect, the invention relates to reducing the formation of by-products during the melt condensation polymerization phase of the preparation of polytrimethylene terephthalate. In a further aspect, the invention relates to the production of polytrimethylene terephthalate which has a reduced tendency to form acrolein when heated in air.
Polytrimethylene terephthalate is a polyester useful in fiber applications in the carpet and textile industries. The manufacture of high molecular weight polytrimethylene terephthalate involves the melt condensation polymerization of 1,3-propanediol and terephthalic acid (or an alkyl ester thereof) to a low molecular weight polyester, followed by solid-state polycondensation to produce a high molecular weight polyester. The 1,3-propanediol can be derived from either acrolein or ethylene oxide, but in each case the product 1,3-propanediol typically contains carbonyl-containing by-products in amounts from 50 ppm to more than 1000 ppm.
The inventor has found that, during the polymerization process, certain of such by-product carbonyls can be converted to acrolein, and their presence in the final product can adversely affect the color of the polymer. Furthermore, when polytrimethylene terephthalate is heated in air at temperatures greater than about 100.degree. C., such as when the polymer is being dried or prior to solid-stating, acrolein is slowly formed. It would be desirable to produce polytrimethylene terephthalate and other 1,3-propanediol-based polyesters in a process which reduced the formation of acrolein and other carbonyl impurities both during polymerization and during heating of the finished polymer in air.
It is therefore an object of the invention to reduce the production of carbonyl-containing byproducts including acrolein during the preparation of 1,3-propanediol-based polyesters. It is a further object of the invention to reduce the tendency of 1,3-propanediol-based polyesters such as polytrimethylene terephthalate to generate acrolein when heated in air.